Ichnofossils (or ichnites) are especially important for clarifying the evolution and prehistoric diversity of taxa. These cannot usually be associated with a particular genus, let alone species of bird, as hardly ever they are associated with fossil bones. But it is possible to group them into ichnotaxa based on their morphology (form). In practice, the details of shape that reveal the birds' behavior or biologic affinity are generally given more weight in ichnologic classification.

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These fossil traces of birds are sometimes hard to interpret correctly, especially when they are from the Mesozoic when the birds' dinosaurian relatives were still in existence. Nests at least of Neornithes are usually quite easy to identify as such due to the unique structures of their eggshells (which incidentally are not trace fossils); there is some uncertainty as regards the origin of certain Mesozoiceggshells, which makes nests of this age problematic.

Mesozoic fossilfootprints are hardest to attribute. "Proto-bird" and related theropod feet were very much alike; non-avian theropod tracks such as the ichnogenus Grallator were initially attributed to ratites because in the early 19th century when these were described, the knowledge about dinosaurian diversity was marginal compared to today, whereas ratites were well-known. Also, under the creationist dogma, scientists would believe that e.g. rheas had been around for all eternity. In the Jurassic and Early Cretaceous, juvenile non-avian theropods left very birdlike footprints. Towards the end of the Cretaceous, the tracks of aquatic birds are usually recognizable due to the presence of webbing between the toes; indeed, most avian ichnotaxa fall into this group. However, giant flightless birds also existed by that time, as evidenced by Gargantuavis; if the Gastornithidae were indeed close to Anseriformes, their lineage must also have been distinct by then. Such footprints may resemble those of non-avian theropod or even ornithopod dinosaurs. Among the former, the Ornithomimiformes (= "Arctometatarsalia" sensu stricto) were convergent to ratites in many respects, including the feet, and it is impossible to tell if some large bird-like footprints from the Late Cretaceous are from an ornithomimiform or a giant bird, without associated bone material.[1]

There exist documented tracks that appear avian since the Late Triassic, by some 55 million years predating the first proper evidence that very birdlike theropods were present. The Late Triassic and early-midJurassic tracks have been assigned to the ichnogenera Trisauropodiscus and Aquatilavipes. Few scientists would go as far though to consider these traces evidence that birds evolved much earlier than generally believed, and perhaps not from theropod dinosaurs as per today's mainstream opinion.

Footprints of at least Neornithes can be distinguished by several features:

if a hallux is present, it is directed straight backwards or nearly so.

the second to fourth (front) toes have a wide angle between them (generally 90-180° or so)

due to Neornithes having a completely fused tarsometatarsus (the "lower leg", actually the ankle and midfoot bones) they have no heel pads (except large terrestrial birds)

It is notable that Heterodontosauridae are known from the localities and times when the first avian-looking footprints started to appear. These small ornithopod dinosaurs were entirely unbirdlike, except for their ornithischianpelvis and a tarsometatarsus strongly convergent to that of Enantiornithes. Though some details remain unresolved, it is far more plausible that Trisauropodiscus etc were made by a Heterodontosaurus-like animal rather than some sort of bird.

Grimaldi, David A. & Case, Gerard Ramon (1995): A feather in amber from the Upper Cretaceous of New Jersey. American Museum Novitates3126: 1-6. PDF fulltext

Patterson, John & Lockley, Martin (2004): A Probable Diatryma Track from the Eocene of Washington: An Intriguing Case of Controversy and Skepticism. Ichnos11(3-4): 341-347. doi:10.1080/10420940490442278